One Oklahoma fire department and a power company in Idaho recently demonstrated how a partly filled water bottle could magnify the sun’s rays and start a fire.

David Richardson, of the Midwest Fire Department in Oklahoma, told KFOR the sunlight “uses the liquid and the clear material to develop a focused beam, and sure enough, it can actually cause a fire.”

“The sunlight will come through (the bottle) when it’s filled with liquid and act as a magnifying glass as you would with regular optics,” said Richardson.

A test at the fire department, outside a car, showed sunlight going through a water bottle raised the temperature of a piece of paper to 250 degrees, KFOR reported.

Representatives from Idaho Power also showed the same potential problem in a Facebook post in July, with a video showing direct sunlight going through a water bottle leaving smoke and burn marks in car seats before the bottle was removed.

While the risk of fire is relatively small, officials recommend keeping water bottles out of unattended vehicles, KFOR reported.